The present invention relates to a demodulator for angle modulated signals having a carrier frequency, the demodulator comprises:
a demodulation channel provided with a local oscillator and with an channel output for supplying a representation of angle modulated signals;
a frequency error detection means coupled to the channel output for deriving a local oscillator frequency error signal; and
a frequency error correction means having two inputs and an output for providing the frequency error corrected angle modulated signals, one input whereof is coupled to the channel output, whereas its other input is coupled to the frequency error detection means
The present invention also relates to a receiver, a transceiver and to a communication device each respectively provided with such a demodulator outlined in claim
Such a receiver is known from U.S. Pat. No. 4,317,210. The known receiver comprises a demodulation channel embodied with a quadrature frequency transposition stage having a local oscillator and with an argument detector connected thereto. At a channel output of the demodulation channel there are connected a frequency error detection means and a frequency error correction means. The frequency error detection means is embodied with a series arrangement of a differentiator, a DC-shift detector and an integrator for supplying a local oscillator frequency error signal, which is representative of a possible frequency shift between the local oscillator frequency and the carrier frequency of received signals which are input to the demodulator. The frequency error correction means is embodied with a difference producer in the form of a subtracter, wherein the local oscillator frequency error signal is subtracted from a signal on the channel output, which signal represents angle modulated signals to yield an subtracter angle modulated output signal which is at least free of the aforementioned possible frequency shift.
It is a disadvantage of the known receiver that the demodulation hardware concept is not applicable to a wider range of in particular the more advanced demodulation schemes.
It is an object of the present invention to provide a receiver, transceiver or other communication device, each comprising a demodulator which is applicable in more sophisticated demodulation schemes also.
Thereto the demodulator according to the present invention is characterised in that the frequency error correction means are embodied as complex plane rotation means for providing complex angle modulated signals by complex plane rotation of a representation of said angle modulated signals and said local oscillator frequency error signals.
It is an advantage of the demodulator according to the present invention that the complex plane rotation means apart from the fact that these rotation means can itself be implemented either analog or digital, allow application in so called Soft Decision Information (SDI) demodulation methods. These are methods for extracting data symbols from in phase (I) or quadrature (Q) down-converted and demodulated angle modulated representative signals on the channel output, such as for example in Zero Intermediate Frequency (Zero-IF) or near Zero-IF demodulators, wherein use is being made of Shift Keying, such as Phase Shift Keying, Frequency Shift Keying, their multi-level variants and the like. The demodulator according to the invention is therefore particularly but not exclusively well suited for application in pagers, mobile telephones and the like communication devices.
An embodiment of the demodulator according to the invention has the characterising features that the demodulator comprises a symbol value determining circuit coupled to the complex plane rotation means. This leads to a simple embodiment of the demodulator as a whole because by simply inspecting the signs of the real and imaginary parts of the complex signal output of said rotation means a data symbol value can be determined in e.g. a slice circuit. For example in the case of a 4-level FSK scheme easy quadrant inspection may be performed to find four possible data symbols.
In a further embodiment the demodulator according to the invention is characterised in that the demodulator comprises a phase velocity estimating means coupled to the channel output of the demodulation channel. Advantageously the result provided is the phase velocity of the signal, which according to a still further embodiment of the invention after integration in an integrator coupled to the phase velocity estimating means yields as the phase of its complex integrator output a signal, which is representative for the modulation frequency xcfx89.
A still further embodiment of the demodulator according to the invention is characterised in that the frequency error detection means are coupled to the channel output through coupling to the output of the frequency error correction means, and that the frequency error detection means comprises a series arrangement of an overdeviation detector and an up/down counter having a counter output which is coupled to the other input of the frequency error correction means. This embodiment advantageously provides an alternative for the arrangement of the frequency error detection means.
At wish in alternative embodiments the complex plane rotation means are implemented either analog or digital. In case of a digital implementation the demodulator according to the invention is characterised in that the complex plane rotation means comprises a processor which is programmed to perform a rotation of input signals in the complex plane. Because of the programmability of the processor this provides additional design flexibility for the demodulator.